1. Field of the Invention
This invention relates generally to the field of multiple channel audio systems. More particularly, this invention relates to a method and apparatus for providing an improved surround sound system having overhead channels to provide a more realistic three dimensional sound which is particularly advantageous to theater applications such as movie theaters.
2. Background of the Invention
Conventional theater surround sound systems include a plurality of speakers distributed across the front of the theater with surround speakers located along each side of the theater. This is illustrated in detail in FIG. 1 which shows an example speaker layout within such a theater. In this arrangement, the movie screen 20 is located on or near a front wall 22. The movie screen typically covers an array of speakers. The array of speakers includes a left channel speaker 24, a left center channel speaker 26, a center channel speaker 30, a right center channel speaker 34 and a right channel speaker 38. Also shown is a sub-woofer 42 which reproduces the extreme lower frequency sounds. Left and right surround speakers 48 and 52 are mounted on side walls 56 and 58 of the theater respectively to produce the effect of surrounding the audience in the seating area 64.
The audio information needed for reproduction of the above channels is optically encoded using, for example, the Sony Dynamic Digital Sound SDDS.TM. format or the Dolby SRD format. In the SDDS.TM. format, digital data representing the eight audio channels are optically encoded between the film perforations and the film edge. Eight channel surround sound can also be encoded using other encoding formats including the methods described in PCT application number WO92/14239 assigned to Dolby Laboratories Licensing Corporation, which is hereby incorporated by reference.
The speaker arrangement of FIG. 1 provides for a realistic surround sound for most situations. However, the two dimensional speaker arrangement of FIG. 1 is best suited for reproduction of two dimensional sound sources. There are many real life situations in which sound comes from above the listener, for example, when an airplane or helicopter passes overhead at low altitude, or the launching of a rocket or missile. This type of sound cannot be completely accurately simulated using the conventional surround arrangement of FIG. 1 with a substantial degree of realism since the listener expects to detect that the source of the sound is overhead. It is therefore desirable to provide for one or more channels of sound to be produced by speakers mounted over the listener's head. Unfortunately, in a theater environment, this must be accomplished under the constraints of the bandwidth available in the 35 mm film format which is common to most theaters.
It is also desirable to provide the improved audio quality accompanying use of digital audio technology as in Sony's SDDS.TM.. Unfortunately, these digital technologies are pushing the bandwidth limits of current motion picture film technology in order to obtain the eight channels of information required to drive the speaker arrangement of FIG. 1. Accordingly, there is a need to provide a mechanism for encoding additional channels without use of additional bandwidth.